Turning your head to look back is a signal.
In slow crowded traffic you need to keep your hands on the handlebars, ready to brake. You can usually move into line with the cars while signaling only with a turn of the head.
Whatever signal you use, always make sure that the driver behind you has noticed your signal and made room for you.

To prepare a left turn, change lanes until you reach the left-turn position in traffic.
As you move toward the center of the street, this is where no cars on your left will go straight ahead. If the lane carrying left-turning traffic also carries through traffic, ride at its left side.
If it's a left-turn-only lane, ride at its right side or in its center, depending on its width.
an ordinary two-lane street, turn left from just to the right of the centerline.

When turning left, don't let left-turning cars behind you pass you on the right.
While waiting, keep near the middle of the lane or make a slow signal with your right hand.
As you enter the intersection, ride straight ahead a short distance so the left-turning cars behind you can pull to your left.

When going straight through, stay out of a right-turn-only lane. Make sure right-turning traffic passes you on your right. If there's a lane marked for right turns and through traffic, ride near its left side. You may sometimes have to merge into the second or third lane from the curb to avoid the right-turning traffic.

Keep to the left of right-turning traffic when going straight through an intersection.
Do not go to the right of traffic unless you are turning right.

When you approach an intersection where cars are waiting for a stop sign or traffic light, never pass the first car. You never know for sure when or in which direction that car will move.
Besides, while you're passing the car, it may hide a pedestrian or other hazard.
The most difficult intersection to ride straight through is the one that looks simplest - on a small, two-lane street. Traffic in the right lane goes in three different directions - right, straight and left! Still, on a street with parallel parking, the empty space between the parked cars and the corner serves as a right-turn lane. Don't wander right, into this space. Keep going straight ahead.
On a street without parking, pull a little farther into the lane to discourage right-turning drivers from passing you on the left. With a little finesse, you can position yourself just far enough from the curb so cars can pass you on the right to make a legal right turn on red.
Some motorists hesitate to pass between a bicyclist and the curb even to make a right turn.